This invention relates to wide-area fire detection and especially to the detection of forest fires.
Wide-area fire detector apparatus as known, e.g., from European Patent Document EP-A1-0298182, serves for the localization of infrared radiation emitted by objects at a temperature in a range of approximately 300.degree. to 1500.degree. C. in a surveillance area extending several kilometers. Such apparatus is particularly suited for the detection of forest fires from a central observation point in a large forested area. Included in such apparatus is a scanning device with azimuthal freedom of movement and with an optical focusing device, e.g., a reflector, for directing infrared radiation from forest fires in a number of detection areas onto a corresponding number of detector elements. Such detector elements are arranged closely spaced in a row perpendicular to the reflector axis. When the detector apparatus is rotated or panned azimuthally and approximately horizontally about an approximately vertical axis, a number of concentric detection areas result which have different elevation or inclination to the horizontal, and which are periodically scanned as the apparatus turns. If a detector apparatus is installed at an elevated location, e.g., on a mountaintop or on a tall mast, an area extending several kilometers can be monitored by a single detector apparatus for infrared radiation originating from forest fires. The site of a fire can be determined and reported by means of a suitable evaluation circuit.
It is a drawback of such known apparatus that the sensitivity of detection decreases with increasing distance, i.e., with decreasing elevation or inclination of a detection area from the horizontal. In other words, detection of a fire is more difficult at a distance than at close range. In accordance with German Patent Document DE-A1-3710265, this disadvantage is avoided when a detector apparatus moves not only azimuthally, but also by periodic variation of the angle of elevation. During such vertical panning movement, the focal length of the focusing device is automatically adjusted, as a function of the angle of elevation, to maintain approximately constant infrared detector resolution in the entire surveillance area. This requires complicated and failure-prone control means with additional movable components. As a result, long-term operation at a remote location is virtually impossible, as the apparatus requires frequent service.
A further disadvantage of such known forest-fire detectors lies in their susceptibility to parasitic infrared radiation from extraneous sources, especially to direct or reflected solar radiation. While the intensity peak of solar radiation lies in the range of visible light, solar intensity in the infrared range, i.e., in the range of thermal radiation from a forest fire, can be strong enough to erroneously trigger a fire alarm signal. Even diffuse light can have such strong infrared component triggering a false alarm.